AT15991U1 - High-temperature component - Google Patents

Abstract

The invention relates to a high-temperature component (1) made of a refractory metal or a refractory metal alloy with a coating (2) for increasing an emissivity, the coating consisting essentially of: tantalum nitride and / or zirconium nitride; and tungsten having a tungsten content between 0 to 98% wt. %.

Description

description

HIGH TEMPERATURE COMPONENT The invention relates to a high temperature component made of a refractory metal with the features of the preamble of claim 1 and a method for producing a high temperature component.

[0002] In many high-temperature applications, heat transfer takes place predominantly through thermal radiation. A determining factor for the thermal radiation emitted at a given temperature is the emissivity, or the emissivity, of the surfaces involved in the heat transfer. The emissivity indicates how much radiation a body emits in relation to an ideal black body.

The higher the emissivity of a surface, the more thermal radiation power a body can emit through this surface.

The same applies to the absorption of thermal radiation power: since the emissivity and absorption capacity of a body are proportional, a body with a high emissivity also absorbs more radiation power than a body with a low emissivity.

It is therefore the endeavor to choose the greatest possible degree of emissivity for technical surfaces over which heat transfer by radiation is to take place. With a high emissivity, the same radiation output can be emitted at a lower component temperature.

This is immediately apparent from the Stefan-Boltzmann law, which, in a modification to gray bodies, indicates the thermally radiated power of a gray body as a function of its temperature:

P = ε (Τ) σ T ⁴ with P the radiated power, ε (Τ) the weighted average emissivity over all wavelengths, σ the Stefan-Boltzmann constant and T the temperature in Kelvin.

[0009] Lower component temperatures are generally favorable with regard to the service life of the component.

[0010] There are various proposals in the prior art for increasing the emissivity of high-temperature components:

US2014041589 (A1) describes a heating conductor which at least partially has a porous sintered coating made of tungsten. The coating is applied using a slurry process. The porous sinter coating increases the emissivity compared to a smooth tungsten surface.

[0012] There are also examples for increasing the emissivity for high-temperature components in other applications:

EP1019948 (A1) describes a coating for an anode of a high-pressure discharge lamp made of a dendritic metal or a metal compound, whereby values for the emissivity (expressed in terms of the emission coefficient) of over 0.8 are to be obtained. Rhenium is mentioned as particularly suitable for this purpose, since it is particularly suitable for producing a dendritic structure.

A similar approach is followed by EP0791950 (A2), according to which fine-grained tungsten is sintered around a tip of a high-pressure discharge lamp.

In DE1182743 (B) the emission coefficient of an anode for a high-pressure discharge lamp is increased via cooling fins and, in one embodiment, additionally via sintered tantalum carbide.

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Patent Office According to these documents, the increase in emissivity compared to an uncoated tungsten anode is primarily achieved by increasing the surface area.

The object of the present invention is to provide an improved high-temperature component and a method for producing the same.

The object is achieved by a high-temperature component with the features of claim 1 or by a method with the features of claim 10. Preferred embodiments are specified in the dependent claims.

The applications considered in this application are applications with operating temperatures of typically 1000-2500 ° C or above. These include, in particular, applications in lighting technology (such as electrodes in high-pressure discharge lamps), furnace technology (such as heating conductors, furnace internals, charging devices, crucibles), and medical technology (such as x-ray rotary anodes).

The components involved with high operating temperatures are referred to in the context of this application as high temperature components.

[0021] Refractory metals or refractory metal alloys are generally used for the high-temperature applications mentioned. Refractory metals in connection with the present application include the metals of the 4th group (titanium, zirconium and hafnium), the 5th group (vanadium, niobium, tantalum) and the 6th group (chromium, molybdenum, tungsten) of the periodic table and rhenium Roger that. Refractory metal alloys mean alloys with at least 50 at.% Of the element in question. Among other things, these materials have excellent dimensional stability at high operating temperatures.

Bare metals generally have a very low emissivity.

The emissivity of tungsten at room temperature in the wavelength range 1700-2500 nm is approximately 0.2.

A generic high-temperature component has a coating to increase the emissivity. The coating can be applied to the entire component or only to parts thereof. According to the invention, the coating for increasing the emissivity is essentially composed of zirconium nitride and / or tantalum nitride and tungsten at a content of 0 to 98% by weight. % educated.

“Essentially” here means that the main constituents are zirconium nitride and / or tantalum nitride and, if appropriate, tungsten. The layer can contain other constituents as well as common impurities in small amounts. For example, oxides or carbides and metallic tantalum or zirconium can be present as impurities. The proportion of the main components zirconium nitride and / or tantalum nitride and optionally tungsten is over 98% by weight. %.

In general, the zirconium nitride corresponds to the chemical ratio formula ZrN and the tantalum nitride to the chemical ratio formula TaN, but it could also be other nitrides or sub- or superstoichiometric compounds with nitrogen. For the sake of simplicity, however, ZrN and TaN are mentioned in the registration.

The coating can either be formed exclusively (except where applicable, the above-mentioned components and impurities) from ZrN and / or TaN. Alternatively, the layer can be up to 98 wt. % Tungsten included.

[0028] According to a first embodiment, the coating is designed as a PVD layer (from physical vapor deposition for physical vapor deposition).

In this case, the coating is produced on a suitable sputter target in a physical vapor deposition process on the substrate (a surface of the high-temperature component). A PVD layer is usually smooth and dense, so it has no pores. To enlarge the surface, the substrate can be coated with a me2 / 19

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Patent office mechanical, chemical or thermal process can be structured.

Alternatively, the coating is designed as a sintered layer. Sintered layer is understood to mean a layer which is obtained by a powder metallurgical coating process. A slurry coating may be mentioned as an example of a powder metallurgical coating process. After the actual application of the coating material in the form of particles, the layer application is consolidated by sintering. A sintered layer is usually porous and has a rough surface.

[0031] The coating is preferably designed as a composite layer of finely divided zirconium nitride and / or tantalum nitride particles and tungsten particles. Composite layer is understood to mean a layer which is composed of a mixture and in which the basic constituents are present in their original solid properties. This feature can be realized in particular if the coating is designed as a sintered layer.

[0032] PVD and sintered layers can be easily distinguished because of the very different surface properties.

Because of the production, a sintered layer preferably has a thickness between 2 pm and 300 pm, more preferably between 3 pm and 100 pm, particularly preferably between 5 pm and 50 pm.

In the case of PVD layers, the thickness can also be significantly lower. Typical thicknesses of PVD layers are between 10nm and 4pm.

[0035] The thickness of the coating is not critical for the function.

The coating is preferably formed on the top side of the high-temperature component. This means that the coating forms the outermost layer on the surface of the high-temperature component. When the high-temperature component is used, this layer is intended to participate in heat transfer by means of radiation.

[0037] Additional layers may be present underneath.

The coating of ZrN and tungsten with a content of ZrN in weight percent between 2 wt. Is particularly preferred. % and 75 wt. % ZrN, preferably between 3 wt. % and 60 wt. % ZrN, particularly preferably between 5 wt. % and 45 wt. % ZrN formed.

Tests by the applicant have shown that a coating made from a mixture of ZrN and tungsten has particularly favorable values for the emissivity. It has surprisingly been found that a coating made from a mixture of ZrN and tungsten has a higher emission coefficient than that of pure ZrN and that of pure tungsten.

The maximum for the emissivity was around 36 wt. % ZrN achieved.

Here, an emission coefficient ε of around 0.8 could be achieved at room temperature. The emission coefficient ε of pure ZrN is about 0.5, the emission coefficient of bare tungsten is about 0.2. It was therefore by no means to be expected that the emission coefficient of a mixture of ZrN and tungsten would have a higher value than the pure forms of the species.

[0042] In addition, this allows a particularly economical representation of the coating, since savings can be made in the substance promoting the emissivity - here ZrN.

In addition, the presence of tungsten in the coating causes good compatibility with the refractory metal, which forms the substrate for the coating.

ZrN is also much cheaper than TaN. That is why the ZrN-based coating is a particularly economical variant.

A layer which increases the emissivity and is based on nitrides is particularly advantageous for applications in which a getter effect for oxygen is desired. From the An3 / 19

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It has been observed that the nitrides absorb oxygen at high temperatures. The high-temperature component can thus be protected against oxidation.

[0046] The coating is preferably porous. By porous it is meant here that the coating has a considerable proportion of pores, for example over 5%. The pores present in the volume of the coating also increase the surface of the coating compared to the purely geometric surface, as a result of which the emissivity is further increased. This characteristic is particularly true for sintered layers.

In one variant, the surface of the high-temperature component is structured below the coating, so that the surface of the coating is enlarged compared to the purely geometric surface. This further increases the emissivity. So here the coating itself is not necessarily porous. The increase in the surface area results from the structuring of the substrate (the high-temperature component). This is particularly relevant if the coating is designed as a PVD layer.

It is preferably provided that the high-temperature component is designed as an electrode of a high-pressure discharge lamp. The application of the coating to an electrode, in particular the anode of a high-pressure discharge lamp, is particularly advantageous. The coating to increase an emissivity on an electrode, in particular the anode, can emit a higher thermal radiation power, which increases the service life. In other words, the electrode formed in this way can emit more heat during operation, which leads to a reduced component temperature.

According to one embodiment, the high temperature component is designed as a heat conductor. In the context of this application, heating conductors are metallic resistance heaters such as are used in heat treatment systems. Heating conductors can be formed from sheet metal, rod material, twisted wire, bundled wire or from wire mesh. In the case of flat heating conductors, that is to say heating conductors whose basic shape comes from a sheet metal, it may be desirable to provide the coating only on that side of the heating conductor which faces an interior of a furnace during operation of the heating conductor.

When applied to a heating conductor, the coating causes the heating conductor to provide a predetermined heating output at a lower temperature. A lower operating temperature of the heating conductor is favorable because, for example, creep can thereby be reduced. The coating for increasing the emissivity on heating conductors, which are used in coating systems, in particular MOCVD systems (from metalorganic chemical vapor deposition), is particularly interesting. Too high a temperature of the heating conductor can lead to evaporation of the base material of the heating conductor (for example tungsten) and consequently to contamination of the substrate to be coated. With a higher emissivity, the heating conductor can be operated at the same heating output at a lower temperature, which reduces the risk of contamination of the substrate to be coated. A particular benefit here is that the vapor pressure of ZrN and TaN is comparable to that of tungsten. This means that a coating based on ZrN or TaN allows the heating conductor equipped in this way to be operated at a lower temperature without the effect of the reduced temperature of the heating conductor being compensated for by a higher vapor pressure of the coating. Overall, a lower operating temperature extends the life of a high temperature component.

According to a further exemplary embodiment, the high-temperature component is designed as a crucible. Crucibles made of refractory metal are used, for example, to melt aluminum oxide in the manufacture of sapphire single crystals. For this, the crucibles are placed in a high-temperature furnace and heated there by heat conductors using radiant heat. The heat transfer mainly takes place via the jacket surfaces of the crucible, which absorb the radiant heat and pass it on to the material to be melted. The coating according to the invention couples a larger proportion of the heat given off by heating conductors into the crucible.

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Patent Office [0052] The high-temperature component preferably consists of at least 98% by weight tungsten. Tungsten has proven to be particularly suitable for the relevant high-temperature components.

Protection is also sought after for a method of manufacturing a high temperature component. According to the invention, the method for producing a high-temperature component comprises the steps:

- Provision of a base body of the high-temperature component, [0055] i) [0056] - Enlargement of a surface of the base body of the high-temperature component, [0057] - Coating of the base body with ZrN and / or TaN and optionally tungsten via physical vapor deposition [0058] or Ii) [0060] coating the base body with powder containing Zr and / or Ta and optionally tungsten using a powder metallurgy process, [0061] heat treatment of the coated base body in a nitrogen-containing atmosphere [0062] or [0063] iii) coating the base body with ZrN and / or TaN and optionally tungsten by means of a powder metallurgy process, heat treatment of the coated base body in a nitrogen-containing and / or argon-containing atmosphere.

The base body is understood to be the high-temperature component or the semifinished product from which the component is produced before coating.

[0067] Three different process variants are therefore proposed.

According to process variant i), a surface of the base body of the high-temperature component is first pretreated in such a way that the surface is enlarged compared to the geometric surface. This “roughening” can be done, for example, with a slurry coating.

In a slurry process, powdery constituents are slurried in a liquid. The suspension obtained, which generally also contains binders, can be used to coat components (here the base body of a high-temperature component) by dipping, spraying or brushing or the like. After drying, the coating is usually sintered. The coating formed in this way is usually porous and rough. It forms a favorable base for a subsequent coating with tungsten and ZrN and / or TaN via physical vapor deposition (PVD). The slurry coating can be based, for example, on tungsten powder.

Alternatively or additionally, the surface can be structured by a mechanical, chemical or thermal method. Blasting, such as sandblasting, may be mentioned as an example of a mechanical method. An example of a chemical process is etching or pickling. Laser structuring may be mentioned as an example of a thermal process.

Thereafter, ZrN and / or TaN and optionally tungsten are deposited on the base body, which has an enlarged surface. A sputtering target with the appropriate composition can be used as the source for the coating. By suitable choice

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Patent office of the target composition, the preferred layer composition can be set. For coatings with ZrN, a composition of the PVD coating of ZrN and tungsten with between 2 wt. % and 75 wt. % ZrN, preferably between 3 wt. % and 60 wt. % ZrN, particularly preferably between 5 wt. % and 45 wt. % ZrN particularly advantageous.

For TaN, a mixture with tungsten brings about a reduction in the emissivity compared to pure TaN, while for ZrN the emissivity of mixtures with tungsten is surprisingly higher than that of the respective pure species.

The enlargement of the surface causes an additional increase in the emissivity beyond the contribution from the coating material.

This process variant (PVD route) can be advantageous if, for example, recrystallization of the base material is to be avoided in order to obtain certain mechanical properties. In addition, this process variant can also prevent warpage of components with narrow component tolerances. The PVD coating takes place at moderate temperatures and does not require any heat treatment step of the coating.

According to process variant ii), the base body is first coated with Zr-containing and / or Ta-containing powder and optionally tungsten using a powder-metallurgical process and then subjected to a heat treatment in a nitrogen-containing atmosphere.

The powder metallurgical process can be a slurry process. Containing Zr or Tahal here means that the powder of the powder metallurgical process contains zirconium or tantalum. It can be, for example, metallic zirconium or metallic tantalum. For zirconium in particular, a dosage form has also proven itself as a hydride. Mixtures of the Zr-containing or Ta-containing powder with tungsten are favorable. This applies in particular to zirconium.

[0077] The layer applied by powder metallurgy is preferably sintered or presintered.

A layer is thus first obtained which contains zirconium or tantalum in elementary or in a compound. In the subsequent heat treatment step of the coated base body in a nitrogen-containing atmosphere, the Zr-containing or Ta-containing species is converted to the respective nitride. According to this process variant, the nitrides are adjusted in situ to increase the emissivity.

The nitrogen-containing atmosphere can be a gas mixture with nitrogen (N ₂ ). Ammonia (NH ₃ ) is also suitable as a nitrogen source.

The desired layer composition can be set by adjusting the ratio of Zr-containing or Ta-containing powder to tungsten. As already stated, a composition of ZrN and tungsten with between 2 wt. % and 75 wt. % ZrN, preferably between 3 wt. % and 60 wt. % ZrN, more preferably between 5 wt. % and 45 wt. % ZrN particularly interesting.

This process variant can be advantageous if, for example, systems for nitriding heat treatment are available anyway.

Because of the availability of plants and above all because of the availability of Zr- and Ta-containing powders, this variant can offer a cost-technical advantage over process variants i) or iii). On the other hand, this process also enables areas to be provided with a nitride-containing layer that cannot be coated using a conventional PVD process (shadowing effects during PVD coating).

According to process variant iii), the base body is coated with ZrN and / or TaN and optionally tungsten by means of a powder metallurgy process and a subsequent heat treatment of the coated base body in a nitrogen-containing one

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Patent office and / or argon atmosphere. The powder metallurgical process can again be a slurry process.

The coating can be carried out either with ZrN or TaN alone, mixtures thereof or with mixtures of ZrN and / or TaN and tungsten.

Mixtures of ZrN and tungsten have proven to be particularly interesting, in particular a mixture of ZrN and tungsten with between 2 wt. % and 75 wt. % ZrN, preferably between 3 wt. % and 60 wt. % ZrN, particularly preferably between 5 wt. % and 45 wt. % ZrN.

In this process variant, the respective nitrides are thus incorporated directly in the powder metallurgical coating step. The heat treatment primarily serves to mechanically consolidate the layer. The atmosphere therefore does not necessarily have to be nitriding.

The heat treatment in a nitrogen or argon-containing atmosphere is preferably carried out at temperatures above 1400 ° C. The nitrogen or argon-containing atmosphere can be a gas mixture with nitrogen (N ₂ ).

This process variant is advantageous because of the simple production. Depending on the availability of plants and raw materials, this variant can be less expensive than the previously presented process variants. In addition, the structure of the layer can be controlled by this variant. The nitrides are processed directly and can be graded or evenly distributed over the entire layer thickness.

Individual process variants are explained in more detail in the following production examples.

PRODUCTION EXAMPLE I:

To produce an improved high-temperature component, tungsten samples were coated with slurries of different powder mixtures.

For this purpose, were first in a binder of 2.8 wt. % Ethyl cellulose in ethanol tungsten or ZrN or TaN powder to a total solids content of 55 ± 2 wt. % weighed in. Stirring was carried out with a Netzsch Multimaster at 1500 rpm for 15 minutes. The mixture was then dispersed in a Bandelin HD 2200 ultrasound homogenizer for 1.5 minutes.

The following layer compositions were examined:

100 wt. % TaN 80 wt. % TaN, balance tungsten 66 wt. % TaN, balance tungsten 50 wt. % TaN, balance tungsten 33 wt. % TaN, balance tungsten 100 wt. % Tungsten 6 wt. % ZrN, remainder tungsten [00100] 9 wt. % ZrN, balance tungsten 13 wt. % ZrN, remainder tungsten 23 wt. % ZrN, balance tungsten 36 wt. % ZrN, remainder tungsten 50 wt. % ZrN, remainder tungsten

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Patent Office [00105] 76 wt. % ZrN, remainder tungsten 100 wt. % ZrN The weight percentages given here relate to the weight of the solid components ZrN, TaN or tungsten.

The weight of 36 wt.% ZrN corresponds to a molar ratio of about 1: 1 based on zirconium and tungsten.

A subsequent spray coating was carried out manually with a distance of approximately 20 cm on tungsten platelets to a target layer mass of 15 mg / cm 2. Drying was carried out with room air.

The dried layer was then subjected to a heat treatment (annealing). In this heat treatment, the organic matter (binder) is first removed and then the layer is consolidated or sintered.

The heat treatment was carried out at 1900 ° C for one hour. To study the influence of the sintering atmospheres, these were varied: it was sintered under argon (Ar), nitrogen (N ₂ ) and high vacuum.

The emissivity of the layers was measured using a Solar 410 reflectometer from Surface Optics Corporation at room temperature. The layers according to the invention were compared with an uncoated tungsten surface and with coatings known from the prior art to increase the emissivity.

[00113] Several wavelength ranges were examined. The results of the measurements in the range between 1700-2500 nm were used to compare the emissivities, since this range is particularly relevant for assessing the thermal radiation of a body.

[00114] A selection of results is summarized in Table 1:

sample Emissivity ε comment 1 Tungsten (blank) 0.21 State of the art 2 Tungsten porous 0.34 State of the art 3 TaN (100% by weight) 0.90 inventively (Embodiment) 4 Tungsten + 36 wt. % ZrN 0.78 inventively (Embodiment)

Table 1: Comparison of the emissivity for different coatings. An uncoated bare tungsten surface, sample No. 1, showed an average emissivity of 0.21 in the examined wavelength range between 1700-2500 nm.

A porous tungsten coating (sample no. 2) obtained by a 100% tungsten slurry had an emissivity of 0.34.

In sample No. 3, TaN (100 wt.%), An emissivity of 0.90 was measured. In this case the coating consisted entirely of TaN, ie without the addition of tungsten. In the preparation of this sample, TaN was applied using a slurry process and then subjected to a heat treatment at 1900 ° C. for 1 hour under N ₂ .

Sample No. 4 had a coating of 36 wt. % ZrN, rest of tungsten. The powder mixture of ZrN and W was applied to the sample using the slurry method and sintered at 1900 ° C. for 1 h under N ₂ . The emissivity was determined to be 0.78.

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PREPARATION EXAMPLE II:

An alternative representation of the coating to increase an emissivity is the production of a sintered layer with Zr or Ta and subsequent nitriding. In the production example, a Ta slurry layer was applied and this was then nitrided in an NH ₃ atmosphere. The nitriding treatment converts at least part of the tantalum into a tantalum nitride. An increase in emissivity compared to a porous Ta layer was found on samples produced in this way.

Analogously, a ZrN layer can be realized by a nitriding treatment of a previously applied W-Zr coating.

PRODUCTION EXAMPLE III:

Another variant for displaying the coating to increase an emissivity is the application of the coating of ZrN and / or TaN by PVD. In the production example, the substrate was first provided with a conventional slurry layer to increase the surface. Pure TaN was deposited on this in this example via PVD. With this coating of 100% by weight TaN, an emission coefficient ε of 0.9 could be achieved at room temperature.

[00122] The invention is explained in more detail below by figures. It shows:

Fig. 1a-1c [00124] Fig. 2a-2c [00125] Fig. 3a-3b [00126] Fig. 4 [00127] Fig. 5 [00128] Fig. 6 [00129] Fig. 7 [00130 ] Fig. 8a-c scanning electron micrographs of TaN-coated surfaces after various annealing treatments scanning electron micrographs of 36 wt. % ZrN, rest of tungsten-coated surfaces after various annealing treatments, scanning electron micrographs of fracture surfaces, a diagram of the emissivity epsilon (ε) for various coatings, schematically a high-pressure discharge lamp as an embodiment of a high-temperature component, a heating conductor as an embodiment of a high-temperature component, a crucible as an embodiment of a high-temperature component, schematically the process of Embodiments of the method according to the invention.

Figure 1a shows a scanning electron micrograph of a TaN-coated surface, which was annealed at 1900 ° C for one hour under a nitrogen atmosphere. The coating was realized by a slurry coating with TaN powder.

[00132] The viewing direction is normal to the coated surface.

In contrast to annealing under argon (Figure 1b) and high vacuum (Figure 1c), the layer remained stable in a nitrogen atmosphere and showed no deformation and smoothing.

A shaping and smoothing by oxidation and sintering was observed under argon and in particular under high vacuum.

For a high emissivity, a porous surface texture such as the TaN surface in FIG. 1a is sought.

The determination of the emissivity ε on the TaN layer after the various annealing treatments resulted in a strong decrease in, in particular for vacuum annealing

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Emissivity to 0.77 compared to 0.90 of the sample annealed under nitrogen.

Figures 2a to 2c show analogous to Figures 1a to 1c scanning electron microscope images of coated surfaces with 36 wt. % ZrN, rest of tungsten, after different annealing treatments.

[00138] The coating was implemented by a slurry coating with ZrN powder and tungsten powder.

The sample in Figure 2a was annealed under N2, the sample of Figure 2b under Ar, the sample of Figure 2c under high vacuum.

The 36 wt. % ZrN, remainder of tungsten samples showed significant indentation and oxidation after annealing under high vacuum. For a high emissivity, a porous surface finish as in FIG. 2a is sought.

Figure 3a shows a scanning electron micrograph of a fracture surface normal to the surface of a sample with a coating of 36 wt. % ZrN, remainder tungsten. The coating was realized by a slurry coating with ZrN powder and tungsten powder.

In the lower part of the picture you can see the substrate made of tungsten sheet material. The coating 2 can be seen above this to increase an emissivity. The porosity of the coating 2 is clearly visible. The porosity further contributes to an increase in the emissivity.

Figure 3b shows a section of a fracture surface of the same sample at a higher magnification. This shows the tungsten particles (“W”) in a matrix made of ZrN. So it is a composite layer of zirconium nitride particles and tungsten particles.

The proportion by volume (measured by quantitative structural analysis) of the particularly advantageous variant is approximately 80% ZrN and 20% W.

Figure 4 shows a diagram of the emissivity epsilon (ε) for different coatings 2 based on ZrN with varying contents of ZrN.

The ZrN content in% by weight is plotted on the horizontal axis (abscissa), and the resulting emissivity epsilon (ε) is plotted on the vertical axis (ordinate). The points in the diagram indicate the respective measured values. The measured value for 0% ZrN corresponds to the emissivity of a bare tungsten surface (ε = 0.21), the measured value for 100% ZrN corresponds to the emissivity of a pure ZrN coating without tungsten (ε = 0.50). A schematic trend line is drawn with dashed lines.

It can be seen that the emissivities of a coating made of a mixture of ZrN and tungsten do not run as expected along a straight line between the values for pure tungsten and pure ZrN, as indicated by the dotted line “s _th ”. Rather, a coating made from a mixture of ZrN and tungsten shows a maximum value in the range of around 36% by weight ZrN. The emissivity is not very sensitive to even lower levels of ZrN, ie even at levels up to about 5% by weight ZrN, attractive high values for the emissivity could still be obtained. At levels above 40% by weight ZrN, however, the emissivity drops sharply. As can be seen from the diagram, a composition of ZrN and tungsten with between 2 wt. % and 75 wt. % ZrN, preferably between 3 wt. % and 60 wt. % ZrN, more preferably between 5 wt. % and 45 wt. % ZrN particularly interesting.

Figure 5 shows schematically a high-pressure discharge lamp 5. Between the electrodes - a cathode 4 and an anode 3 - a discharge arc is formed during operation. In the present exemplary embodiment, the anode 3 is the high-temperature component 1 and is provided with a coating 2 according to the invention to increase an emissivity.

Due to the coating 2, the anode 3 can emit a higher thermal radiation power, which increases the service life.

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AT15 991 U1 2018-10-15 Austrian Patent Office [00150] The cathode 4 or both the anode 3 and the cathode 4 can also be provided with the coating 2.

Shown here by way of example on a high-pressure discharge lamp 5, the coating 2 can also be used for other lamp types to increase an emissivity.

Figure 6 shows a heat conductor 6 made of a refractory metal in an exemplary arrangement as a floor heater of a high-temperature furnace. The heating conductor 7 is heated by direct current passage and heats the interior of the high-temperature furnace by emitting radiant heat.

In the present exemplary embodiment, the heating conductor 6 forms the high-temperature component 1 and is provided with a coating 2 according to the invention to increase an emissivity. When used on a heating conductor 6, the coating 2 has the effect that it can produce a predetermined heating output at a lower temperature. This reduces creeping of the heating conductor 6 and extends the service life.

Figure 7 shows schematically a crucible 7 made of refractory metal. Crucibles made of refractory metal are used, for example, to melt aluminum oxide in the manufacture of sapphire single crystals. For this, the crucibles are placed in a high-temperature furnace and heated there by heat conductors using radiant heat. The heat transfer takes place mainly via the surface of the crucible, which absorbs the radiant heat and passes it on to the material to be melted. The crucible 7 forms the high-temperature component 1 in the present exemplary embodiment and is provided with a coating 2 according to the invention to increase an emissivity.

The coating 2 in use on a crucible 7 causes a larger proportion of the heat given off by heating conductors to be coupled into the crucible 7. The crucible 7 thus reacts more quickly to heat input from heating conductors.

The application of the coating 2 is in no way limited to the examples shown here. The coating 2 is generally advantageous for high-temperature components at which heat transfer is to take place by means of radiation.

Figures 8a-c schematically show the sequence of exemplary embodiments of the method according to the invention.

Figure 8a shows the sequence of process variant i).

On the left half of the picture, the base body of the high-temperature component 1 is shown, the surface of which has been enlarged by a treatment. The measure of increasing the surface area serves to increase the emissivity.

According to the above picture, the surface was enlarged by applying a slurry layer. In the example, the slurry layer was applied with tungsten powder ("W"). Instead of a tungsten slurry, other slurry compositions compatible with the substrate can also be used. After the slurry coating, the powder application is sintered, which is not specifically shown.

According to the image below, the surface was enlarged by mechanical, chemical or thermal structuring.

In both cases of the embodiment, the substrate, that is, the base body of the high-temperature component 1, is made of tungsten.

The main body of the high-temperature component 1 is then coated with tungsten and ZrN and / or TaN via physical vapor deposition. The sputtering process is indicated schematically in the center of the image via a sputtering target 8. The target 8 can either consist of the components of the layer, or alternatively the nitride can also be formed during the process.

The result, the high-temperature component 1 with a coating 2 to increase 11/19

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Patent office hung emissivity, is shown in the right half of the picture. The composition of the PVD layer can be determined by the choice of the sputter-target composition. The PVD layer is usually only a few nm or a few pm thick.

Figure 8b shows the sequence of process variant ii).

In the left half of the figure it is shown that the base body of the high-temperature component 1 receives a coating with a Zr-containing and / or Ta-containing powder and optionally tungsten via a powder metallurgical process. An example of a powder metallurgical process for coating is a slurry process.

In the middle picture, the subsequent heat treatment of the coated base body of the high-temperature component 1 is shown in a nitrogen-containing atmosphere. The letter "N" in the indicated heat treatment device 9 symbolizes the nitrogen-containing atmosphere. The heat treatment converts zirconium or tantalum into the corresponding nitrides and consolidates the coating.

As shown in the right half of the figure, a high-temperature component 1 with the coating 2 is obtained to increase the emissivity.

Figure 8c shows the sequence of process variant iii).

Shown on the left is the base body of the high-temperature component 1 with a layer applied by means of a powder metallurgy process with ZrN and / or TaN and optionally tungsten.

In the middle picture, the subsequent heat treatment of the coated base body of the high-temperature component 1 is shown in an atmosphere containing nitrogen and / or argon. The letter "N" in the indicated heat treatment device 9 symbolizes the nitrogen-containing atmosphere, "Ar" the argon-containing atmosphere. The coating is consolidated by the heat treatment.

As shown in the right half of the figure, a high-temperature component 1 with the coating 2 is obtained to increase the emissivity.

LIST OF REFERENCE NUMBERS USED:

High-temperature component

Coating to increase emissivity

anode

cathode

High pressure discharge lamp

heating conductor

crucible

Sputtering target

Heat treatment facility

12/19

AT15 991 U1 2018-10-15 Austrian

Patent Office

Claims (19)

Expectations 1. High-temperature component (1) made of a refractory metal or a refractory metal alloy with a coating (2) to increase an emissivity, the coating essentially consisting of: tantalum nitride and / or zirconium nitride; and tungsten with a tungsten content between 0 to 98% by weight. %. 2. High-temperature component (1) according to claim 1, wherein the coating (2) is designed as a PVD layer. 3. High-temperature component (1) according to claim 1, wherein the coating (2) is designed as a sintered layer. 4. High-temperature component (1) according to claim 1 or 3, wherein the coating (2) is formed as a composite layer of tantalum nitride and / or zirconium nitride particles and tungsten particles. 5. High-temperature component (1) according to one of claims 1 to 4, wherein the coating (2) made of zirconium nitride and tungsten with between 2 wt. % and 75 wt. % Zirconium nitride is formed. 6. High-temperature component (1) according to one of the preceding claims, wherein the coating (2) is porous. 7. High-temperature component (1) according to one of the preceding claims, wherein the coating (2) is formed on the top side of the high-temperature component (1). 8. High-temperature component (1) according to one of the preceding claims, wherein the high-temperature component (1) is designed as an electrode (3, 4) of a high-pressure discharge lamp (5). 9. High-temperature component (1) according to one of claims 1 to 7, wherein the high-temperature component (1) is designed as a heating conductor (6). 10. High-temperature component (1) according to one of claims 1 to 7, wherein the high-temperature component (1) is designed as a crucible (7). 11. A method for producing a high-temperature component (1) with a coating (2) to increase an emissivity, comprising the steps: - Providing a base body of the high-temperature component (1), i) o enlarging a surface of the base body of the high-temperature component (1), o coating the base body with ZrN and / or TaN and optionally tungsten via physical vapor deposition or ü) o coating the base body with Zr-containing and / or Ta-containing powder and optionally tungsten via a powder metallurgical process, o heat treatment of the coated base body in a nitrogen-containing atmosphere or iii) o coating the base body with ZrN and / or TaN and optionally tungsten via a powder metallurgical process, o heat treatment of the coated base body in a nitrogen-containing and / or argon-containing atmosphere 12. The method according to claim 11, wherein the enlargement of the surface of the base body of the high-temperature component (1) in sub-step i) takes place via a slurry coating of the base body. 13/19 AT15 991 U1 2018-10-15 Austrian Patent Office 13. The method according to claim 11, wherein the enlargement of the surface of the base body of the high-temperature component (1) in sub-step i) takes place via a mechanical, chemical or thermal structuring of the base body. 14. The method according to claim 11, wherein in sub-step ii) or sub-step iii) the coating of the base body is carried out using a slurry process. 15. The method according to any one of claims 11 or 14, wherein the heat treatment in substeps ii) or iii) is carried out at temperatures above 1400 ° C. 5 sheets of drawings 14/19 AT15 991 U1 2018-10-15 Austrian patent office 15/19 AT15 991 U1 2018-10-15 Austrian Patent Office 16/19 AT15 991 U1 2018-10-15 Austrian patent office 17/19 AT15 991 U1 2018-10-15 Austrian Patent Office 18/19 AT15 991 U1 2018-10-15 Austrian Patent Office W / ZrN / TaN 19/19